1. Field of the Invention
The present invention relates to an optical data recording/reproducing device for recording a data in and reproducing a data from an optical disk, and an integrated head containing a light source and photodetectors for the optical data recording/reproducing device.
2. Description of Related Art
As an optical head for writing a data in and reading data from an optical disk is generally used a separate optical type head, in which a movable unit having a minimum size and weight necessary for high speed access is separated from a fixed unit.
FIG. 1 is a schematic perspective view of a conventional separate optical type head. An optical disk 3 is mounted so as to rotate in a horizontal plane (i.e., the x-y plane). Below the optical disk 3 (i.e., in the negative direction of the z-axis) a movable unit 2 of the optical head is disposed movably along the positive and negative direction of the y-axis. An integrated head 1, which is part of a fixed unit of the optical head, is disposed away from the movable unit 2 in the positive direction of the y-axis (i.e., in the radial and centrifugal direction of the optical disk 3).
A coherent light source and photodetectors are built in the integrated head 1 as described below. The integrated head 1 projects a beam emitted by the light source in the negative direction of the y-axis, and the beam is reflected by a prism 2a of the movable unit 2 in the positive direction of the z-axis. The reflected beam is then converged by an objective lens 2b so as to be projected on the optical disk 3. The beam reflected by the optical disk 3 is allowed to proceed through the same optical path in the reverse direction to reach the integrated head 1. Rolling bearings 2d of the movable unit 2 are in contact with and roll on a rail 4 extending in the y-axis direction. A driving force to move the movable unit 2 in the y-axis direction is obtained by using a coil 2c provided to the movable unit 2 and a magnetic circuit 5 disposed in the direction of the y-axis so as to be combined with the coil 2c.
The main functions of the optical head include focus servo and track servo. The focus servo is a control for focusing a light beam on a recording surface of the optical disk 3, and the track servo is a control for allowing a focal point to track a desired track on the optical disk 3. The principles of such control will now be described. In the following description, it is assumed for simplification that the light source and the photodetectors are disposed below the optical disk 3 without using the prism 2a. FIG. 2 is an elevation view of such an optical head taken in the positive direction of the x-axis. A laser beam emitted by a laser diode 1a, the coherent light source, is projected through a holographic optical element 15 and the objective lens 2b onto the optical disk 3. A reflected beam from the optical disk 3 proceeds in the reverse direction and enters the holographic optical element 15 so as to be diffracted and deflected by a micro angle in accordance with a grating pattern formed on the holographic optical element 15. The thus deflected light enters photodiodes 14a, 14b, 14c and 14d that, are disposed, as the photodetectors, on the both sides of the laser diode 1a (i.e., along the positive and negative direction of the y-axis).
Before describing the focus servo using the holographic optical element 15, the principle of a Foucault method and a focus error signal FES used in the focus servo will be described first.
When the face of the optical disk 3 is deviated due to the rotation of the optical disk 3, the data recording film thereon is moved in the z-axis direction. A beam spot emitted through the objective lens 2b is servo-controlled so as to follow the movement and be always focused on a data recording film on the optical disk 3. For this servocontrol, a focus error signal (FES), whose output value is varied in accordance with a relative distance between the objective lens 2b and the optical disk 3, is used. An example of a method for obtaining the FES includes the Foucault method, in which one quartered photodiode or two halved photodiodes are used.
FIGS. 3 through 5 illustrate the relative distance between the optical disk and the objective lens together with a spot image formed on the photodiode, and FIG. 6 is a graph showing the relationship between the relative distance and the FES. When the optical disk is positioned at the focal point as is shown in FIG. 4, the reflected light from the optical disk forms elliptical spot images on a quartered photodiode 17, and the approximate center of the elliptical spot is located on a halving line CL1. The FES is obtained by using outputs AA, BB, CC and DD from the four regions A, B, C and D of the quartered photodiode as follows: EQU FES=(AA+CC)-(BB+DD)
When the light spot is correctly focused, the FES is 0.
When the optical disk is nearer than the focal point, the beam pattern is formed on the photodiode as two semicircles in the regions A and C as is shown in FIG. 3, and the value of the FES is positive in this case. When the optical disk is farther than the focal point, the beam pattern is formed as two semi-circles in the regions B and D as is shown in FIG. 5, and the value of the FES is negative in this case. In this manner, the output value of the FES is positive when the relative distance is short and negative when it is long, as is shown in FIG. 6. The focus servo can be realized by utilizing the FES having such a characteristic.
Next, the track servo will be described. When the optical disk is decentralized due to the rotation thereof, the track on the optical disk is shifted in the y-axis direction. Therefore, servo is required so that-the beam spot emitted through the objective lens 2b follows the shift of the track and is always focused on the track. For this purpose, a track error signal (TES), whose output value is varied in accordance with the relative distance between the beam spot and the track, is used. An example of methods for obtaining the TES includes a push-pull method, wherein a halved photodiode is used. FIGS. 7 through 9 illustrate the relative distance between the beam spot and the track together with a spot image formed on the photodiode, and a reflected light intensity distribution is also shown in each of the drawings. When a beam spot converged by the objective lens is positioned rightly on the track, the intensity distribution of the light reflected by the optical disk is symmetrical about an optical axis OA. The light intensity distribution on the halved photodiode is also symmetrical about a halving line CL2.
The TES is obtained by using outputs AA and BB of the regions A and B of the halved photodiodes as TES=AA-BB. When the beam spot is positioned on the track, the TES is 0. When the beam spot is shifted to the left of the track as is shown in FIG. 7, the intensity of the reflected light is higher on the left side of the optical axis OA. Therefore, also in the intensity distribution on the halved photodiode, the intensity on the left side of the halving line CL2 is higher, and the TES is positive in this case. When the beam spot is shifted to the right of the track as is shown in FIG. 9, the TES is negative. The track servo can be realized by using the TES having such a characteristic derived from the positive and negative shift of the beam spot. The beam pattern formed on the photodiode due to the rightward or leftward shift of the beam spot from the track is designated as a push-pull pattern.
Now, the methods for the focus servo and the track servo using the holographic optical element 15 shown in FIG. 2 will be described.
FIG. 10 is a schematic plan view of the optical head of FIG. 2 taken in the positive direction of the z-axis. The holographic optical element 15 is divided into two regions 15a and 15b in the y-axis direction so that the frequencies of the diffraction gratings in the regions 15a and 15b are different from each other: the former has a shorter frequency and the latter has a longer frequency. The halved photodiodes 14a and 14b are located so as to receive the+primary diffracted light from the regions 15a and 15b, respectively. Because of the length difference in the grating frequencies as mentioned above, the photodiode 14a is positioned farther in the y-axis direction than the photodiode 14b. Similarly, the halved photodiodes 14d and 14c are located so as to receive the--primary diffracted light from the regions 15a and 15b, respectively.
More specifically, the photodiode 14a is positioned as follows: The imaging point by the +primary diffracted light from the region 15a, which is obtained when the focal point is located on the optical disk 3, is located on a halving line 14a.sub.3 of the photodiode 14a. Similarly, the photodiode 14b is positioned so that the imaging point by the +primary diffracted light from the region 15b, which is obtained when the focal point is located on the optical disk 3, be located on a halving line 14b.sub.3 of the photodiode 14b. Under this condition, when the output of a region 14a.sub.1 of the halved photodiode 14a is taken as AA, the output of a region 14a.sub.2 as BB, the output of a region 14b.sub.1 of the photodiode 14b as DD, and the output of a region 14b.sub.2 as CC, the FES can be calculated as (AA+CC)-(BB+DD) on the basis of the Foucault method as described referring to FIGS. 3 through 6.
The halving lines on the photodiodes 14c and 14d receiving the -primary diffracted light extend in the x-axis direction. Each region of the photodiodes 14c and 14d receives the light at the intensity distribution as described referring to FIGS. 7 through 9, and hence, the TES can be calculated as AA-BB. Although one photodiode is sufficient for this purpose, two photodiodes connected in parallel can double the output, resulting in improving the quality of a signal to be outputted.
In conducting the focus servo and the track servo as described above, the halving lines 14a.sub.3 and 14b.sub.3 of the photodiodes 14a and 14b extend in the y-axis direction. Further, the push-pull patterns corresponding to the spot images on the photodiodes 14a and 14b are halved by the halving lines 14a.sub.3 and 14b.sub.3, respectively.
The reasons for the above will be described. First, the reason for the halving lines 14a.sub.3 and 14b.sub.3 extending in the y-axis direction is as follows: The oscillation wavelength of a laser beam emitted by the laser diode 1a is varied due to various causes. For example, when the recording/reproducing device is shifted from the reproducing operation to the recording operation, the power of the laser diode is generally increased from 4 through 5 mW to 27 through 30 mW. At this point, the wavelength of the laser diode is varied to be longer by approximately 2 through 5 nm. In the holographic optical element, since the light beam is deflected based on the principle of diffraction as is shown in FIG. 11A, the angle of diffraction increases from .theta. 0 to .theta. 1 when the wavelength becomes longer.
In accordance with this change of the angle of diffraction, an elliptic beam spot 21a.sub.1 formed on the photodiode 14a changes its position as a beam spot 21a.sub.2 as depicted in FIG. 11B. When the photodiode 14a is positioned so that the beam spot changes its position along the halving line 14a.sub.3, the light beam intensity at the regions 14a.sub.1 and 14a.sub.2 is not varied by this positional change of the beam spot. As a result, the value of the FES is not varied. When this positional change is made not parallel to the halving lines 14a.sub.3 and 14b.sub.3, however, the following problem occurs: When the beam spot changes its position due to the variation of the wavelength of the laser diode, the beam spot moves diagonally about the halving line 14a.sub.3, and the light beam intensity at the regions 14a.sub.1 and 14a.sub.2 is varied, thereby varying the value of the FES. As a result of the variation of the FES, the beam spot is defocused. This also goes for the photodiode 14b.
Next, the reason for the push-pull pattern corresponding to the spot image on the photodiode being vertically halved by the halving line is as follows:
FIGS. 12A, 12B and 12C illustrate the relationship between the halving lines of the holographic optical element and the photodiodes and the push-pull pattern. In FIG. 12A, a halving line 15c of the holographic optical element 15 is substantially parallel to a push-pull pattern 24. In this case, the tracking direction is parallel to the y-axis. A reference numeral 22 denotes an intensity distribution in the x-axis direction of reflected light from the optical disk 3 entering the holographic optical element 15. The distribution 22 is obtained when the track is shifted in the positive direction of the x-axis from the beam spot. The light having entered the holographic optical element 15 is halved by the halving line 15c. Light diffracted in the region 15a as described above is converged on the photodiode 14a as a beam spot. 21a depicted in FIG. 12C. At this point, a light intensity distribution 23a in the positive direction of the x-axis is equally divided as is shown as portions 23a.sub.1 and 23a.sub.2 in FIG. 12C.
Further, light beam diffracted in the region 15b is converted on the photodiode 14b as a beam spot 21b as depicted in 12B. A light intensity distribution 23b in the positive direction of the x-axis in this case is not equally divided as is shown as portions 23b.sub.1 and 23b.sub.2 (i.e., 23b.sub.1 &gt;23b.sub.2). When the outputs of the regions 14a.sub.1, 14a.sub.2, 14b.sub.2 and 14b.sub.2 and 14b.sub.1 are taken as AA, BB, CC and DD, respectively, the following relationship holds: EQU 23b.sub.2 &gt;23b.sub.1 &gt;23a.sub.1 =23a.sub.2 EQU .thrfore.CC&gt;DD&gt;A=BB EQU .thrfore.FES=(AA+CC)-(BB+DD)&gt;0
Therefore, the value of the FES is not 0. This means that an offset is caused in the FES, resulting in causing the defocus of the beam spot. Accordingly, such a configuration is inappropriate.
FIGS. 13A, 13B and 13C illustrate another relationship between the halving lines of the holographic optical element and the photodiodes and the push-pull pattern, in which the halving line 15c of the holographic optical element 15 is vertical to the push-pull pattern 24 as depicted in FIG. 13C. In this case, the tracking direction is parallel to the x-axis direction. The reference numeral 22 denotes an intensity distribution in the x-axis direction of reflected light from the optical disk entering the holographic optical element 15. This distribution is obtained when the track is shifted in the positive direction of the y-axis from the beam spot. A reference numeral 25 denotes an intensity distribution in the y-axis direction of the reflected light from the optical disk entering the holographic optical element 15. The light beam that has entered the holographic optical element 15 is halved by the halving line 15c.
The light beam diffracted in the region 15a is converged on the photodiode 14a as the beam spot 21a as described above and depicted in FIG. 13A. The light intensity distribution in the x-axis direction obtained in this case is uniform as is shown as the distribution 23a. The light beam diffracted in the region 15b is converged on the photodiode 14b as the beam spot 21b depicted in FIG. 13B. The light intensity distribution in the x-axis direction in this case is uniform as is shown as the distribution 23b. Under this condition, when the outputs of the regions 14a.sub.1, 14a.sub.2, 14b.sub.2 and 14b.sub.1 are taken as AA, BB, CC and DD, the following relationship holds: EQU 23b.sub.1 =23a.sub.1 =23a.sub.2 =23b.sub.2 EQU .thrfore.AA=BB=CC=DD EQU .thrfore.FES=(AA+CC)-(BB+DD)=0
Thus, the FES takes a value of 0 in this case. This means that neither offset is caused in the FES nor defocus is caused in the beam spot.
As is apparent from the above, in the case where the beam spot is shifted rightward or leftward from the track and the resultant intensity variation of the push-pull pattern is superimposed on the reflected light from the optical disk, neither an offset is caused in the FES nor the beam spot is defocused, if the holographic optical element 15 is disposed with the halving line 15c vertical to the push-pull pattern. In other words, when the holographic optical element 15 is thus disposed, the FES is not disturbed by a push-pull signal.
Considering the above-described fact, it is necessary to allocate the laser diode 1a and the photodiodes 14a, 14b, 14c and 14d shown in FIG. 2 in the y-axis direction so that the halving lines of the photodiodes 14a and 14b for the FES be vertical to the push-pull pattern. In the description referring to FIG. 2, however, it is assumed that the optical source and the photodetectors are both disposed below the optical disk. Therefore, the laser diode 1a and the photodiodes 14a, 14b, 14c and 14d are actually allocated in the z-axis direction as is shown in FIG. 1. When these elements are disposed in this manner, the size of the data recording/reproducing device corresponding to the thickness direction of the optical disk tends to be large. An attempt to minimize the size of the device in the z-axis direction has been a technical problem in common, regardless of the mounting direction of the optical disk.